In certain situations, as when repairing dams, it is necessary to make diaphragm walls to a depth of more than 100 meters.
The operating technique for making such walls is similar to that for making conventional diaphragm walls. An excavation is made in the ground and filled with a liquid known as “mud”, and generally based on bentonite. The mud forms a waterproof deposit on the walls of the excavation, thus enabling it to control percolation through the ground and prevent the walls collapsing. Once the excavation has reached the desired depth, it is progressively filled with concrete, beginning underneath the mud, in the bottom of the excavation.
In order to reach the desired concreting depth and in the particular situation of so-called “deep” concreting, it is common practice to use a concreting column constituted by a plurality of column elements or segments assembled to one another.
The various segments are assembled together progressively as the column is lowered inside the excavation.
Concreting begins when the bottom end of the column is close enough to the bottom of the excavation.
The first step, generally referred to as “priming”, consists in filling the concreting column with concrete by replacing the mud that was initially present with concrete, but without polluting the concrete with mud.
After concrete has filled a predetermined volume of the excavation, the feed of concrete to the column is stopped, the column is raised through a height substantially equal to the length of a segment, and its top end segment is removed. This operation is referred to as “shortening” the column.
While concreting is stopped in order to shorten the column, pressures inside the column and in the remainder of the excavation are balanced, with the top surface of the concrete inside the column moving down. When concreting is restarted, concrete poured into the inlet of the column drops through air over a considerable height. This drop, and/or sudden contact between the newly poured-in concrete and the concrete at the bottom of the column can give rise to segregation of the concrete and possibly to the formation of a plug inside the column, thereby preventing operations from being continued.
Dropping concrete over a large height can also lead to air being held captive under pressure inside the column, which can lead to concrete being discharged from the top end of the column while the air is being expelled, thereby constituting a potential risk for the safety of operators, or from the bottom end of the column, thereby leading to a reduction in the quality of the concrete.
The problem of concrete drop height also arises during the priming stage, and independently of any operations of raising the column, as soon as the feed of concrete to the column is not continuous.